1. Field of the Invention
The present invention relates generally to a diode detection circuit, and more particularly, to a diode detection circuit suited for high frequency signal detection.
2. Description of the Related Art
Detection circuits or rectifiers using diodes are known. FIG. 1 depicts an example of the configuration thereof. A parallel circuit consists of a resistor R1 and a capacitor C1 and is connected to the cathode side of a diode D1.
When an input signal has a small high-frequency voltage level such as in the case where a diode detection circuit of FIG. 1 is used to detect the high-frequency signal level between cellular phones and base stations, the forward voltage drop of the diode D1 will become unnegligible.
That is, the input high-frequency voltage is sagged (voltage dropped) by the forward voltage of the diode D1 and is output across the resistor R1. Due to the nonlinearity in the voltage-current characteristics of the diode D1, the detection linearity may become degraded in the minute high-frequency voltage area such as the high-frequency signals between the cellular phones and the base stations.
In addition, the influences of the temperature characteristics of the diode D1 may cause a large temperature variation in the detection characteristics. The forward voltage fluctuations attributable to the temperature variations may vary depending on the temperature at the rate of about xe2x88x922 mV/xc2x0 C., and hence xe2x88x92100 mV output voltage fluctuation will occur by variation of 50xc2x0 C.
Thus, a diode detection circuit of FIG. 2 having improved detection linearity and temperature variations was proposed (xe2x80x9cDiode Detection Circuitxe2x80x9d disclosed in Japan Patent Laid-open Pub. No. Hei7-111421). The diode detection circuit of FIG. 2 is configured so that an operational amplifier OA1 offsets the voltage drop and fluctuations of the detection diode D1 by the voltage drop and fluctuations of a compensation diode D2.
The detection diode D1 and the compensation diode D2 are correlated with resistors R1 and R2, respectively, so that the ratio of the resistor R1 to the resistor R2 conforms to the ratio of the saturation current value of the detection diode D1 to the saturation current value of the compensation diode D2. This allows different types of diodes to be used as the diodes D1 and D2.
FIG. 3 shows another diode detection circuit in which the operational amplifier OA1 is disposed on the input side of the detection diode D1, with the output of the detection diode D1 being fed back to the negative input of the operational amplifier OA1 to allow an action as the diode D1 with zero forward voltage. It is to be noted that if the FIG. 3 circuit remains unvaried, the circuit output is given in the form of a pulsation current. Similar to the examples of FIGS. 1 and 2, the addition of the capacitor to the output side enables the high-frequency voltage peak value to be held.
The above circuit of FIG. 2 aims to relieve the voltage drop and temperature variations of the diode D1 by rectifying the high-frequency voltage by the diode D1 and thereafter correcting it by the operational amplifier OA1 and the diode D2.
In this circuit, however, as shown in FIG. 4, when the diode D1 rectifies the high-frequency signals, only a half-cycle current WC1 flows therethrough whereas the diode D2 allows a flow of direct current DC1 therethrough. Thus, the diodes D1 and D2 make remarkably different dynamic actions.
As is apparent from FIG. 5 showing the relationship between the high-frequency voltage WC1 and the surge current SC, in case of the capacitor input detection circuit as depicted in FIG. 2, the surge current SC flows through the diode D1 for a brief period of time near the peak value of the high-frequency voltage, to charge the capacitor C1. Then, gradual discharge is made till the next half cycle (PV in the diagram), and again the charging is effected near the next peak value. These operations are iterated. Accordingly as the mean value of the DC voltage becomes closer to the peak value of the AC voltage WC1, the time during which the surge current flows will decrease and the ratio of the surge current SC to the mean direct current becomes larger.
The time rate during which the surge current SC flows depends on the product of the internal resistance of the diode D1 and the cathode side external capacitance, and will be of the order of {fraction (1/20)} of one cycle. The direct current DC flowing through the diode 2 is about {fraction (1/12)} of the surge current SC.
Therefore, irrespective of the same internal resistances of the diodes D1 and D2, there lies a large difference between the surge current SC and the direct current DC flowing therethrough, with the result that the diodes D1 and D2 may disadvantageously suffer different forward voltages.
Another problem lies in that it is difficult to faithfully provide the input high-frequency voltage peak value PV as the DC output.
On the contrary, the circuit of FIG. 3 is a circuit which is introduced as an ideal diode detection (rectification) circuit, although the high-frequency signals must be amplified by the operational amplifier OA1 since the output of the operational amplifier OA1 is rectified by the diode D1. It may suffer a further drawback that the operational amplifier OA1 must have an enough higher (ten times or more) bandwidth than the high-frequency signal to be rectified from the viewpoint of through-rate.
Naturally, good performance of the operational amplifier OA1 may merely result in an ideal circuit of FIG. 3. It may not be easy however even to make gain amplification at a high high-frequency voltage of 1 to 2 GHz. In addition, amplification of the high high-frequency voltage of 10 to 20 Ghz with a high gain will lead to significantly increased costs.
As discussed above, the conventional example circuits of FIGS. 2 and 3 tend to suffer the above respective deficiencies.
It is therefore the object of the present invention to solve the above problem involved in the conventional diode detection circuit and to provide a diode detection circuit capable of obtaining an ideal rectification voltage even in the case of a minute input high-frequency voltage.
In order to achieve the above object, according to an aspect of the present invention there is provided a diode detection circuit comprising a first diode that accepts an AC signal; a first parallel circuit consisting of a resistor and a capacitor, the first parallel circuit accepting a detection output from the first diode; a first operational amplifier having a positive input terminal that accepts a charging voltage for the capacitor of the first parallel circuit; a second diode that accepts an output from the first operational amplifier; a first switching circuit consisting of a first switch and an oscillator, the first switching circuit providing a control of the ratio of conduction to non-conduction of the first and second diodes; a second parallel circuit consisting of a resistor and a capacitor, the second parallel circuit accepting an output from the first switching circuit, with a charging voltage for the capacitor of the second parallel circuit being applied to a negative input terminal of the first operational amplifier; and a holding circuit that holds an output from the first operational amplifier.
Preferably, the diode detection circuit further comprises a second switching circuit disposed on the input side of the holding circuit, the second switching circuit effecting its switching operations alternately with the first switching circuit, and the holding circuit holds a positive peak value of the output from the first operational amplifier.
Preferably, in the diode detection circuit, the holding circuit holds a negative peak value of the output from the first operational amplifier. Preferably, the diode detection circuit further comprises a positive power source exceeding in the forward direction of the first diode.
Preferably, the diode detection circuit further comprises a negative power source connected to a positive input terminal of the first operational amplifier, the negative power source exceeding in the forward direction of the first diode.
Preferably, the first switching circuit approximates the second diode conduction time to the first diode conduction time.
Preferably, the resistors of the first and second parallel circuits have the same resistance value or approximate resistance values, and the ratio of the peak value of the charging current to the mean value of the discharging current of the first diode conforms to the ratio of the peak value of the charging current to the mean value of the discharging current of the second diode by causing the product of the resistor, capacitor and the input frequency of the first parallel circuit to conform to the product of the resistor, capacitor and the oscillator frequency of the second parallel circuit.
The above and other objects, aspects, features and advantages of the present invention will become more apparent from the embodiments of the present invention which will be described in conjunction with the accompanying drawings.